The levels of naturally-occurring arsenic in the world's drinking water are apparently so high that it's considered a global health risk. This news comes as scientists discover that the problem is much larger than they originally suspected -- with up to 140 million at risk in 70 different countries.

In places where the contamination is most-prevalent, it's thought that hundreds of thousands of people could be killed. Elsewhere, arsenic is causing lung disease and cancers -- even after people have stopped drinking contaminated water.

According to one researcher, "most countries" have at least some water sources with "dangerous levels of arsenic," -- and scientists are only just now beginning to understand how far-reaching this problem really is.